Concrete Without Steel

The Marine Mortar Secret

Eli: In 2013, Marie Jackson and a group of fellow researchers had Roman harbor concrete on the lab bench, as Berkeley Lab told it. The samples were drill cores from a project called ROMACONS, sponsored by the Italian cement maker CTG Italcementi of Bergamo. The team ran them through electron microscopy, using beams of electrons to magnify tiny details, and X-ray diffraction. Those harbor structures had spent more than two thousand years in the sea, taking saltwater, chemical attack and waves the whole time.

Mara: And they're still there. Which is exactly what the listener asked: how come there are still Roman and Greek structures standing after thousands of years when modern buildings fall apart after decades? Today's angle is the paradox of ancient durability versus modern decay.

Eli: Which raises the obvious suspicion. If the sea couldn't wreck those harbors, maybe the Romans had a way of building that's since been lost. Nobody's settled that yet.

Mara: Pliny the Elder already bragged about it: "impregnable to the waves and every day stronger".

Eli: Every day stronger is close to what Jackson was chasing. The Roman recipe was lime mixed with pozzolana, a volcanic ash rich in silica and alumina. It isn't filler. It reacts.

Mara: It keeps working after the pour.

Eli: Keeps working for centuries. Jackson said "what we consider corrosion processes can actually produce extremely beneficial mineral cement and lead to continued resilience."

Mara: Here's how that works. Jackson's group also used synchrotron X-ray microdiffraction, an intense beam that maps minerals across spots a few micrometers wide, plus electron microscopy and Raman spectroscopy. By their account, seawater seeped through the concrete and dissolved bits of volcanic crystal and glass. New minerals grew in their place: phillipsite, and aluminous tobermorite, a rare crystal that forms long flat plates that interlock, like shingles laid in every direction. Those plates gave the submerged concrete an unusual flexibility, and it increased with time.

Eli: She described going back and finding "abundant tobermorite growing through the fabric of the concrete." And the ash wasn't random. Around 27 BC, when Augustus launched a citywide program to repair old monuments and build new ones, builders used ash from just one deposit, Pozzolane Rosse, from the Alban Hills volcano southeast of Rome. Jackson and her colleagues say that ash's mix of minerals appears to have helped the concrete withstand chemical decay.

Mara: A recipe chosen by its source, then, not a lucky accident.

Eli: Chosen, and in the harbors, soaked. But most of Rome never sat in the sea, Mara, and seawater was doing a lot of the work in those cores.

Self-Healing Mortar and Corroding Steel

Mara: On land it was rain and groundwater instead. The very thing modern concrete is meant to keep out.

Eli: Start at the tomb of Caecilia Metella, on the Appian Way. MIT scientists and colleagues studied its concrete. Over the centuries, rain and groundwater moved through it and dissolved leucite, a volcanic mineral in the aggregate, the gravel and stone chunks mixed into concrete. That released potassium. Instead of forming damaging gels, it reorganized and steadied the binding phases that hold the concrete together. One researcher said they saw binding domains "that were intact after 2,050 years."

Mara: Which may leave her tomb in better shape than her male contemporaries' monuments.

Eli: Then, in 2023, Admir Masic's team looked at mortar from a well-preserved wall at Privernum. The cracks ran right through lime clasts, little white lumps of quicklime that never fully dissolved during mixing. Water gets into a crack, dissolves a clast, and calcium carbonate recrystallizes and fills the gap. They tested it. Within two weeks, a cracked sample had healed and water could no longer flow through. An identical sample made without quicklime never healed.

Mara: Masic says hot mixing the quicklime also sped construction up.

Eli: Self-sealing walls, made with no modern chemistry. That's the reverse of what gets poured today.

Mara: The exact reverse. By itself, concrete is a very durable material. The weak point in modern concrete is the steel inside it. Paulo Monteiro, an engineer at UC Berkeley, says "Corrosion of steel reinforcement is the main cause of concrete deterioration, so comparisons should be made with great care." Water and salt work their way in, the bars rust and swell, and the concrete around them cracks and breaks off in flakes. That's spalling.

Eli: And reinforced-concrete structures typically last 50 to 100 years, sometimes less.

Mara: Set that beside a tomb that's been soaking up rain for two thousand years. On those numbers, it looks as if the Romans simply built better, everywhere they built.

The Tenement Illusion

Eli: Everywhere is where it breaks. Most people in imperial Rome lived in insulae, crowded rental apartment blocks several stories high. A 4th-century catalogue of the city counted 46,602 of them.

Mara: Against under 1,800 private houses. That's the city most Romans actually lived in.

Eli: And plenty of it was shaky. Some insulae were solidly built, but many weren't: weak materials, poor maintenance, collapse, fire. The poet Juvenal wrote that Rome was "held up for the most part by slender Props," and that once management had "covered some ancient yawning Crack, they'll tell us to sleep soundly at the edge of ruin."

Mara: And those blocks are exactly what's missing from the ruins.

Eli: Missing because they fell or were torn down. That's survivorship bias: judging a whole group by the few that happened to last. A 2025 review warned that studies of ancient materials may carry it, since only the most durable Roman structures remain. It cuts the other way too: Portland cement has only about 200 years of real-world record. So Jackson's harbor cores had already passed a two-thousand-year test before anyone drilled them. The question of lost technology looks different once the failures are counted.

Mara: And the survivors share a shape. Roman mass concrete had no steel and little strength in tension, so builders relied on massive arches, vaults and domes that work in compression, everything pressing inward and down. Sara Conti puts it this way: "Roman cement isn't 'better' than Portland — it's optimised for a different problem: lasting forever in pure compression, with no reinforcement to protect."

Eli: No reinforcement to protect. So nothing inside to rust.

Mara: Which is why the evidence points to a skewed record: fragile rental blocks gone, massive compressive monuments left, many of them looked after for centuries. What failed can't be sampled, because it isn't there anymore. And Greek temples had no Roman concrete at all, Eli, yet some still stand.

The Marble Mountain and the Titanium Cure

Eli: And on the Acropolis they managed it without mortar, too.

Mara: Classical temples are post and lintel: columns standing up, stone beams laid across them. The stone is dry-joint masonry, blocks cut to fit so tightly that friction holds them, with no mortar in between. The whole temple sits on dressed bedrock. For extra hold, builders added iron clamps and dowels set in lead. Friction first, metal second.

Eli: Iron inside stone, though. That's the rebar problem in miniature.

Mara: Reinforcing steel bars. Except the ancient iron mostly held. Jackson and Masic say ancient clamps and dowels had a "remarkable capacity" to withstand corrosion through the ages.

Eli: The marble had a rougher time. In 1687 an explosion hit the Parthenon and pushed columns out of place. One on the south side was knocked so far that, three centuries on, it still threatened to collapse, despite a patch after 1842 that reused a block from a demolished mosque. So in February 1992, restorers moved that column to repair and complete its bottom drum. It weighed 60 metric tons.

Mara: So precision kept it up, and people kept catching it when it slipped.

Eli: Lots of people. Since 2011, restorers have been taking blocks down, repairing them and resetting them, on the west end and the north wall of the inner chamber. That came to 117 blocks. They pulled out rusting iron clamps from earlier repairs, the same trouble Monteiro described in concrete, and put in titanium clamps, a metal that doesn't corrode, so it can't crack the stone from inside. Charalambos Bouras put the rule this way in a 2008 documentary: "we preserve all the original pieces and we add only a few marble in order to fit them to the general construction."

Mara: A temple from the fifth century BC, held together partly by a modern metal. Stone can outlast empires, and that still doesn't explain why so many new buildings are gone within a single lifetime.

The Economic Demolition Cycle

Eli: In April 2024, at a forum on sustainable cement, Marie Jackson turned the question around. The Romans, she said, lacked our energy resources, so they made materials that could maintain themselves. "They couldn't rebuild the Theatre of Marcellus every 30 years because the concrete failed. We have a throw away construction material mentality." Then there's the Pantheon, dedicated under Hadrian. It has the world's largest unreinforced concrete dome, intact and in use for nearly two thousand years. Admir Masic says it "would not exist without the concrete as it was in the Roman time."

Mara: And that dome never had to fight its own steel.

Eli: Modern buildings often don't get the time to fight theirs. Surveys of demolished buildings in Europe and the US put the average age at demolition at roughly 70 years. One 2025 study found 71. What usually ends them is land economics, turnover, a new masterplan. That throws away embodied carbon, the emissions already spent extracting, making and hauling the materials. A 2023 study argued that measuring it in national regulation would "seem highly likely to become a key driver towards the retention of buildings."

Mara: So a lot of falling apart is actually being taken down.

Eli: And people read all of this two ways. Jackson and Masic see real chemistry: concrete that heals and strengthens with water, an advantage modern mixes lack. Jackson has even said the originals may have been "built so well that, once they were in place, they didn't need to be replaced." The other reading, Mara, is the one Monteiro and Conti hold: the survivors are a small, heavily maintained elite, ordinary buildings perished, and modern structures are engineered for planned economic lives.

Mara: And the chemistry hits limits when it meets steel. Concrete is everywhere, and its poor durability makes a large environmental footprint worse. But a 2025 review says Roman concrete gains strength slowly at first, isn't standardized, and has a lower pH. That last part matters: modern concrete's alkalinity forms a chemical shield on the steel, called passivation, and Roman mixes may not provide it. Making Roman concrete releases about as much carbon dioxide per volume, too. An engineer at Universidad del Norte said, "When we take concrete's service life into consideration, that's when we start seeing benefits."

Eli: The Parthenon showed the same split: no rebar to rot, still decades of repair.

Mara: So here's the answer for the listener. The Roman and Greek buildings still standing are mostly massive, unreinforced stone and concrete that only ever had to carry compression, some bound with volcanic mortars that grew crystals or sealed their own cracks, many repaired and kept in use. Most Romans lived in tenements that are gone. Modern buildings carry steel that rusts, and many are pulled down for economic reasons long before they wear out.

Eli: Back to those ROMACONS cores: the crystals were real, and now the question is whether they travel. Two things to watch. Engineering trials of Roman-inspired mixes, and whether they reach early strength and protect steel. And more digs testing how widely and consistently the Romans used hot mixing across the empire, which is still an open question.

Mara: One topic to explore further is pozzolana.

Eli: Follow science with us across more stories as researchers discover why materials survive.

Mara: That's our story for today. I'm Mara.

Eli: And I'm Eli. Thanks for listening to Angle, and until next time.